The present invention relates to a method of measuring the aberrations of a lens and, more particularly, to a method of measuring the aberrations of the projection lens of a reduction-type projection exposure tool.
A reduction-type projection exposure tool is used in the manufacturing process, especially in the lithography step, for a semiconductor device. Various methods of measuring the aberrations of the projection lens of this tool have recently been proposed. Some methods are actually used to estimate aberrations.
Typical aberrations of projection lenses are spherical aberration, astigmatism, and coma.
Spherical aberration is a phenomenon in which the best focus shifts depending on the size of a pattern to be transferred. According to a typical method of measuring spherical aberration, for example, the best focus for a 0.25-.mu.m pattern and the best focus for a 2.0-.mu.m pattern are measured. The difference between the measured best focuses is set as a measure that indicates the magnitude of spherical aberration. If, however, the size of a pattern to be transferred is large, the pattern can be resolved in a wide focus range. That is, since a large best focus range is allowed, it is difficult to determine an accurate best focus.
Astigmatism is a phenomenon in which the best focus shifts depending on the direction of a pattern to be transferred. As a method of measuring astigmatism, the method described in SPIE Vol. 1463 (1991), p. 294 is generally used. In this method, vertical, horizontal, and oblique patterns are exposed by micro-stepping while the focus condition is gradually changed. The focus conditions in which the patterns are resolved best in the vertical, horizontal, and oblique directions are compared by using an optical microscope. As in the case with the estimation of spherical aberration, in this method, as the size of a pattern to be transferred increases, high-precision estimation tends to be more difficult. Therefore, this method is effective for only fine patterns near the resolution limit.
Coma is a phenomenon in which the transfer position shifts depending on the size of a pattern to be transferred. According to a general method of measuring coma, since the size of a 5-bar pattern becomes different between its two ends after transfer, this size difference is used as a measure that indicates the magnitude of coma. Recently, a method of measuring the relative transfer position deviation of large and small patterns by using an overlay inspection tool has been used (Japanese Patent Application No. 9-305917).
The above conventional methods of measuring aberrations are generally used to estimate the influences of aberrations on actual device patterns. For this reason, as illumination and exposure conditions, conditions used to expose actual device patterns are used. In addition, as measurement marks to be formed on a substrate, marks close to the design rules of actual device patterns are formed.
According to these methods, however, since various aberrations influence each other in a complicated manner, each obtained estimation result is no more than relative estimation, e.g., "the influences of aberrations on a pattern B are smaller than those on a pattern A" or "the aberrations of an exposure tool A are smaller than those of an exposure tool B".